One big reason more moms (and dads) don't "lean in" at the office is that they just don't want more work.

As Catherine Rampell at the New York Timeshasbeen reminding us for the past week, that's true for the majority of workers. According to the Families and Work Institute, just 37 percent of working women and 44 percent of working men said they wanted more responsibility at the office in 2008, the last year of data (see below).

Those figures got me wondering, though: When, exactly, do women and men stop trying to climb the corporate ladder? And why? Is it just about having children or is it something else?

To find out, I asked the Institute to break down its 2008 findings by age group, which produced the graph below. It tells a simple story: By our mid-to-late-20s, the desire to take on more responsibility fades fast for both men and women.

In other words, ambition starts sliding right around the time most Americans start having kids. (The median woman has her first child at around age 26). And though women slide a bit further than men, both sexes become less interested in a promotion as they age.* That's right in keeping with what Pew has found about the converging roles of mothers and fathers. At some point in our 30s, most of us lean back.

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*To try and figure out whether this was directly related to motherhood, I took at look at the Institute's numbers comparing mothers under 44 with non-mothers. Unfortunately, the differences they showed weren't statistically significant, thanks to the sample sizes.